High frequency quenching may be applied to harden a workpiece, such as a steel material, to increase the strength of the metal. As mechanical characteristics vary corresponding to the depth of a quench hardened layer, quenching is carried out under preset process conditions, and quality inspection is further carried out after the manufacturing. Conventionally, an example of a method of inspecting whether appropriate quenching is applied to the workpiece or not includes a method of cutting and inspecting an arbitrarily extracted workpiece. In the method, much time is required for the inspection, and the workpiece to be inspected cannot be used as a product. Further, a problem that inspection on all of the workpieces cannot be performed exists. Therefore, a method of nondestructive inspection of a quenching state of a workpiece has been investigated.
Document Cited 1 adopts a method in which permeability is measured on a workpiece made of carbon steel in an axis-symmetrical shape by passing through an induction coil, quenching depth in each cross section of the workpiece is estimated, a surface hardness tester is used to measure surface hardness, and a position with a significant increase in the surface hardness is detected to specify an end point of a quenching range to thereby inspect a quenching pattern of the workpiece.
Document Cited 2 relates to a nondestructive measurement method of depth of a quench hardened layer of a steel material, wherein a low frequency AC magnetic field generated by an excitation coil magnetizes the steel material in a direction along the surface to generate eddy current, a detection coil detects an induction magnetic field induced by the eddy current, and an output voltage of the detection coil is compared with known data to estimate the depth of the quench hardened layer of a symmetrical steel material.